As a process for obtaining expansion-molded articles of plastics, Japanese Patent Application Laid-Open Nos. 195131/1982 and 51123/1983 each disclose a process in which foamed particles are filled into a mold and then heated to fusion-bond the particles to one another, thereby producing a molded article in a desired shape. As a process for producing foamed particles used in molding, Japanese Patent Application Laid-Open Nos. 4738/1986 and 223347/1991 each disclose a process in which particles of a polyolefin resin are impregnated with a volatile organic foaming agent or an inorganic gas type foaming agent in a state that the particles have been dispersed in an aqueous medium in a vessel, the impregnated particles are heated to a temperature not lower than the foaming temperature of the resin while maintaining the internal pressure of the vessel at the vapor pressure of the foaming agent or higher, and the particles are then released from the vessel into an atmosphere of a pressure lower than the internal pressure of the vessel, thereby expanding the particles.
When an inorganic gas type foaming agent is used as the foaming agent, however, it is generally difficult to provide foamed particles having a high expansion ratio. As disclosed in Japanese Patent Application Laid-Open Nos. 4738/1986 and 223347/1991, it has been known to add aluminum hydroxide or borax to a base resin to produce foamed particles in order to enhance the expansion ratio of the resulting foamed particles. However, such foamed particle have involved a problem that when a molded article is produced with the foamed particles, the resulting molded article undergoes significant shrinkage upon molding and the dimensional stability of the molded article is thus deteriorated. Such foamed particles have also involved a drawback that their cells become larger or smaller than they need, so that the cell size varies widely, and this scattering of the cells comes out on the surface of the resulting molded article to impair the appearance of the molded article. More specifically, the regular fusion-bonded state of foamed particles generally appears on the outer surface of an expansion-molded article. However, an irregular fusion-bonded state appears on a molded article obtained by molding foamed articles of uneven cell diameters, so that regularities arise in the external appearance of the molded article. In particular, when cells in foamed particles are significantly fined, such foamed particles are whitened. When such whitened foamed particles are mixed into a raw material for molding, the external appearance of the resulting molded article becomes significantly uneven (irregular), resulting in reduction in its commercial value.
Besides, when carbon black or another coloring pigment is added to foamed particles in advance to color foamed particles, their color looks thin at portions where cells are fine, and thick at portions where cells are large. Therefore, when a molded article has been produced by using, as a raw material, such foamed particles of uneven cell diameters, there has been a drawback that color irregularity markedly occurs on the surface of the molded article to impair the external appearance of the molded article, and so its commercial value is likewise reduced.
It is an object of the present invention to provide foamed particles of a polyolefin resin, which have a high expansion ratio and are free of scatter of cell diameters.
Another object of the present invention is to provide foamed particles of a polyolefin resin as a raw material for molding, from which molded articles undergoing little shrinkage upon molding and hence having excellent dimensional stability can be provided.
A further object of the present invention is to provide foamed particles of a polyolefin resin, from which molded articles exhibiting a regular fusion-bonded state of the foamed particles and having a uniform external appearance can be produced.
A still further object of the present invention is to provide a process of producing foamed particles of a polyolefin resin, by which the above-described foamed particles of the polyolefin resin can be produced with ease.